I know this sounds strange but fecal matter (stool) microbiotia transplants are gaining credibility for treatment of constipation in Parkinson's and Dr. Borody, an Australian doctor had the symptoms of 3 patients diminish/go away afterwards:

"The therapeutic potential of FMT in non-gastroenterologic conditions, including autoimmune disorders,[10] neurological conditions,[11] obesity, metabolic syndrome and diabetes,[12] multiple sclerosis,[11] and Parkinson's disease[13] are now being explored. As of May 2008, studies had shown that FMT can have a positive effect on devastating neurological diseases such as Parkinson's disease.[13] While Dr. Thomas Borody was experimenting with patients who were afflicted by both CDI and Parkinson's disease, he realized that after fecal therapy the symptoms of Parkinson's in his patients began to decrease; some to the point that the Parkinson's could not be detected by other neurologists. The hypothesis for future studies is that the fluctuation in the body's microbiome done by FMT can also be recreated by adding anti–Clostridium-difficile antibodies to the patient's body, a technique intended to be used in Borody's future case studies involving Parkinson's disease.[12]."

Interesting. I think I'd try deploying the "anti–Clostridium-difficile antibodies" option before trying the FMT implant. It appears that there are 2 specific toxins (Toxin A and Toxin B) to target. I'm guessing that either or both may be the suspected culprit and both would need to be targeted...